Low-resistance and high-strength copper direct bonding in no-vacuum ambient using highly (111)-oriented nano-twinned copper

Abstract

In this study, we fabricated (111)-oriented nt-Cu microbumps with 30 mu m in diameter, and bonded them together using chip-to-chip scheme in N-2 ambient, without vacuum. A well bonded interface in the Cu-to-Cu joint was identified by focused ion beam (FIB). Scanning electron microscope (SEM) images showed a void-less bonding interface within the bonded Cu joint. In addition, a die shear test was conducted. The test results revealed that the Cu joint has a robust bonded Cu joint and the shear strength is 124 MPa, which is nearly two times higher than the traditional SnAg solder joint (64 MPa). In addition, fracture analysis showed that the joint fractured in a ductile manner. Besides, we also performed the resistance measurement by using Kelvin probes on the bonded chip-to-chip test vehicles. The resistance is 4.12 m Omega for a single joint resistance and 4.26 x 10(-8) Omega.cm(2) in contact resistivity. More than 30% resistance reduction has been confirmed as compared to the traditional SnAg solder joint (6.32 m Omega). Moreover, we can further reduce the joint resistance by the post-annealing process. It can be brought down to 3.27 m Omega with a resistivity of 3.14 x 10(-8) Omega.cm(2). There is a nearly 50% resistance reduction, as compared to SnAg solder joint. The resistance value after the second annealed Cu joint is close an ideal Cu joint. In summary, low-resistance and high-strength copper direct bonding in no-vacuum ambient using highly (111)-oriented nano-twinned copper has been successfully achieved.